U.S. Pat. No. 5,388,583, issued Feb. 14, 1995 to A. Ragauskas, et al., entitled “Method and Apparatus for Non-Invasively Deriving and Indicating of Dynamic Characteristics of the Human and Animal Intracranial Media”, describes a technique that use the phase and higher harmonic characteristics of the transmitted ultrasound signal to measure changes in the brain tissue. This method is based short-pulsed ultrasonic signals and the time measurements are based on the time elapsed between the transmitted signal and the received signal. This elapsed time, together with some phase shift, is used to provide an estimate of the total travel time of the fast-rising short acoustic pulse. This method does not permit an accurate measurement of travel time, density, dispersivity and classification as it uses single estimated travel time and does not consider the cases of multiple frequencies.
German Patent DE 103 24 990 to M. Wrobel, et al., issued Nov. 4, 2004, entitled “Vorrichtung zur Untersuchung eines flüssigen oder gasförmigen Mediums” (Device for the Investigation of a Liquid or Gaseous Media), describes an ultrasonic interferometer. However, the interferometer described therein does not accommodate the concept of multiple frequencies at the same time. It must scroll through multiple frequencies, which prevents it from monitoring fast processes. Furthermore ultrasonic interferometer described therein utilizes the phase of the signal only and cannot overcome the issue of the 360 degree phase shift. Finally, it is not a scalable system, in that the measurements are based on the geometry and characteristics of each individual sensor. This means that the profiles learned from one system cannot be transferred to another system because the sensor characteristics will be different. The system described in German Patent DE 103 24 990 must relearn everything when it is initialized the first time.
German Patent Application DE 100 27 827 to M. Wrobel, et al., filed Jun. 5, 2000, published Dec. 6, 2001 and entitled “Adaptive Klassifikationseinrichtung” (Adaptive Classification Apparatus) is directed towards a classification system for ultrasound FM signals passed through human tissue. The classification system uses a time varying signal spectrum, i.e. a spectrogram, to achieve classification. In the present invention, classification is achieved by measuring travel times.
PCT application WO204/CA01401 to M. Wrobel and S. Stergiopoulos, published Feb. 3, 2005, entitled “Non-Invasive Monitoring of Intracranial Dynamic Effects and Brain Density Fluctuations”, describes an ultrasonic interferometer that requires at least two transmitting pulses for its operation. Furthermore, the concept of “beat-frequency phase” as described herein is simply the difference of the phase shifts from two successive transmissions, and is not used as a technique to accurately determine the time from a pair, or pairs of phases. Consequently, it suffers from the cyclic nature of the phases, and cannot differentiate φ from (φ+2π). Furthermore, it has that shortcoming that the frequency window that can be used is limited by the thickness of the skull bone. Furthermore, the classification is based solely on the difference of the two phases.